Fifty years after Richmond, Va., native Arthur Ashe won the first U.S. Open, the American men enter this hyped anniversary on the outside looking in as the tournament begins Monday at Flushing Meadows at 11 a.m.

While the USTA is fiercely promoting Ashe and the 50th anniversary of the U.S. National Championships admitting pros and thereby changing its name to the U.S. Open, little buzz exists for any U.S. men on the court. Again.

It’s no wonder thousands upon thousands of European tourists will stroll across the U.S. National Tennis Center grounds these next two weeks.

But there is one hope. A 6-foot-10 serving machine from Georgia, veteran John Isner is seeded 11th coming off his Wimbledon semifinal marathon and leads the modest American men’s brigade. And just maybe the tennis cognoscenti are sleeping on Isner.

“I think they are and I love it,’’ one of Isner’s coaches, Justin Gimelstob, told The Post. “He loves playing on U.S. soil, loves being the No. 1 American. He’s a true American athlete and loves representing his country and being the face of American tennis. It’s a huge opportunity to make a run. He has a very fair draw and can do things you can’t defend against.’’

It’s been 12 years since an American man, Andy Roddick, made it as far as the Open semifinals. Consider all four women’s Open semifinalists last year were American and the differential is stunning. The last American man to make any Grand Slam final is also Roddick (Wimbledon, 2009).

Surprisingly, Isner was not one of the slew of players who attended Friday’s Media Day at Louis Armstrong Stadium. Apparently, the USTA is prouder of its new roofed Armstrong Stadium than any of its men.
To boot, Isner’s opening match is out on the Grandstand, the third showcourt, when he faces American wild-card Bradley Klahn.

At 33, Isner is amid a career rally. Gimelstob and many tennis insiders believe he would have won Wimbledon if the stodgy All-England Club’s rules contained fifth-set tiebreakers. Isner, whose weakness is speed and service return, lost in a ridiculous 26-24 fifth set in the semifinals to Kevin Anderson.

“Think about that rule [costing Isner the crown] 100 times a day — and for the rest of my life,’’ Gimelstob said.

Isner is surprised his career is on the upswing. Now he has to worry whether the impending birth of his daughter — his wife is due Sept. 22 — will interfere with a long Open run.

“I was always a later bloomer,’’ Isner told the New York Times two weeks ago in Toronto during the Rogers Cup. “I was always so big, it took me a long time to grow into my body. I always knew that my best tennis was going to be in my late 20s. Ten years ago I would have been lying if I said I knew it was going to be at 33. But it’s encouraging to see.”

Isner’s resurgence occurred after a four-day minicamp in Los Angeles conducted by Gimelstob, a New Jersey native, and his two other coaches. It got him back on track after a slow start to 2018.

“It was about getting away from the tournament atmosphere, do technical work, reps in the right areas where it’s important,’’ Gimelstob said. “You have to train John differently and we cleaned up his footwork — everything’s more compact, in sync.’’

Frances TiafoeGetty Images

“I think I’ve been able to feel, physically, probably the best I’ve felt in 11 years on tour, which is cool to think about,’’ Isner told reporters recently. “I still do believe that my best tennis is ahead of me.”

In the men’s game, 30 is the new 20, so maybe there is promise for some young U.S. hopefuls. But the wait is still on. Frances Tiafoe, up to No. 42, is by far the most promising of the bunch. Sam Querrey, who made the 2017 Wimbledon semifinals, is not expected to be a factor, his ranking down to 35.

The 25-year-old Jack Sock has fallen off the singles map despite John McEnroe calling him this week the world’s best doubles player. After losing in the first round at Wimbledon to Matteo Berrettini in July, he shook his opponent’s hand and uttered a profanity to him about his coach.

“If I was a stockbroker and Sock was stock, I’d buy in,’’ said Gimelstob, Tennis Channel’s top analyst. “I don’t think it can get any lower. I’d buy stock in a few of those American guys — [Taylor] Fritz, [Reilly] Opelka, Tiafoe.’’

And looking up, 10 American men are in the top 100.

“There are more American men in the U.S. Open draw than any other country, and more American men in the top 100 that any other country,’’ said tennis historian/former USTA spokesman Randy Walker. “But having a guy break through, reach a major final has been the challenge since Andy Roddick retired.”